1. Field of the Invention
The invention relates to a control device of an vehicular automatic transmission and, more particularly, to an improvement in the technology of discharging air that has intruded in a hydraulic circuit of a friction engagement device.
2. Description of the Related Art
In a vehicular automatic transmission which has a plurality of friction engagement devices that are engaged when supplied with oil pressure and which establishes a plurality of speed change steps in accordance with the engaged and released states of the friction engagement devices, air intrudes into a hydraulic circuit of a friction engagement device that is put into the released state during a run of the vehicle, as the working oil is agitated by vibrations of the vehicle or the like. Therefore, if the traveling time with the engagement device being in the released state becomes long, the accumulation of air changes the characteristic of oil pressure change at the time of engagement (response or the like) so that the shift controllability at the time of the shift that involves the engagement of the aforementioned friction engagement device deteriorates and therefore a shift shock (a drive force change or the like due to racing-up, tied-up, etc.) may occur.
Automatic transmissions that include air discharge means for automatically discharging the air that has intruded into the hydraulic circuit of a friction engagement device as described above (air bleeding) are described, for example, in Japanese Patent Application Publication No. JP-A-2002-213595 and No. JP-A-2002-227982. In any one of such automatic transmissions, when the automatic transmission is in the first speed state immediately following the initial N→D shift switch during a state of the vehicle driving where an ignition switch has just been turned on, oil pressure is supplied to the friction engagement device that is released in the first speed so that the air that has intruded in the hydraulic circuit is discharged by the oil pressure. Besides, in Japanese Patent Application Publication No. JP-A-2002-213595, the time of execution of an air discharging process is set in accordance with the working oil temperature so as to make the air discharging process time as short as possible and therefore limit its effect on the shift control. In Japanese Patent Application Publication No. JP-A-2002-227982, the line pressure is increased for a predetermined time from the start of the air discharging process so that the air discharge is efficiently performed.
However, in the case where during the state of the first speed in the D range, oil pressure is supplied to the friction engagement device that is released in the first speed in order to perform the air discharging process, if accelerator operation is performed during that process, the response at the time of launching the vehicle can be impaired due to the need to drain the oil pressure of the friction engagement device, or drag can occur in that friction engagement device due to the transmission of power prior to the complete release of the friction engagement device.